1. Field of the Invention
The present invention relates to a circuit to cancel a DC offset appeared in a differential circuit, and a differential circuit providing this offset canceller.
2. Related Prior Art
A differential circuit, especially a circuit applied in an optical communication system, is necessary to amplify a signal with a broadband width from low frequencies, typically 1 kHz, to high frequencies, typically 1 MHz. Such a differential circuit usually provides a DC offset canceller including a low-pass filter or an integrator accompanied with a capacitor. Because of its large capacitance, the capacitor in the low-pass filter or the integrator often configures as an external capacitor, which inevitably forces the IC to provide additional two lead pins for the external capacitor. When the capacitor for the low-pass filter or the integrator is set within the IC, the die area must be enlarged, which raises the cost of the IC.
A Japanese Patent published as 2001-274640A has disclosed a circuit to increase the capacitance able to be built within the IC equivalently by using the Miller effect inherently observed in the inverting amplifier. However, this circuit varies the trans-conductance gm of the transistor depending on the gate bias, which also changes the gain of the inverting amplifier and the capacitance obtained by the Miller effect. Applying this offset canceller to the differential circuit for the optical communication system, the instability of the input bias due to a change of the ambient and operational temperature and a drift of the power supply or the input signal with relatively low frequency components change the Miller capacitance of the inverting amplifier, which makes the cut-off frequency of the filter or the integrator instable; thus the signal quality in the lower frequencies is degraded.